The present invention relates to a method of inspecting surfaces and, in particular, it concerns a method for inspecting surfaces having a periodic pattern such as dies and cells that are produced on the surface of silicon wafers used in the Integrated Circuits (IC) industry.
Many known inspection methods for defect detection are based on a comparison principal. According to this principal the gray levels of each pixel in the digital image acquired from the inspection region are subtracted from the gray levels of their corresponding pixels in a digital reference-image related to the inspection region. A defect is indicated if the subtraction results for any pixel of the digital image are greater than a predetermined threshold value.
When the acquired image of the inspection region contains a large amount of pixels due to one or a combination of the size of the inspection region or high resolution of the image, the reference-image has to be stored in a large volume memory which is very expensive.
For some applications such as high-resolution inspection of silicon wafers, the cost of large volume memory makes the use of the above methods impractical. An alternative inspection method is also based on a comparison technique and is useful for surfaces having a periodic pattern. In this method, the scan is performed along stripes or bands known as swaths. The swaths are aligned with the periodic structure of the periodic pattern. Since the pattern contains many periodic fragments, such as cells or dies and the swaths are relatively narrow, the information in the swath relating to each periodic fragment is dramatically reduced. As the inspection area has a periodic pattern, the surface can be inspected by making a comparison between swaths that are in related fragments of the periodic pattern. Accordingly, this comparison method requires a relatively small memory-volume and it also eliminates the need for a reference-image.
A difference detected by a comparison between two fragments indicates that one of the fragments is defective, but the defective fragment is not identifiable. Three-fragment comparison is needed to identify the fragment and the defect location within the identified fragment and not just to detect the existence of a defect without the ability to indicate the exact location of the defect. Three-fragment comparison is performed by comparing the fragment under inspection with two adjacent fragments. Statistically, it is assumed that there is a very low probability that a defect will repeat itself at the same position in two other fragments. Therefore, a defect is defined as a deviation that appears twice in the two comparisons and the fragment that contains the defect is the one that differs from the other two fragments. The three-fragment comparison method is also known as Cell-to-Cell or Die-to-Die comparison. The three-fragment comparison method is only effective when the scan direction of the swaths is aligned with a periodic structure that typically has Cartesian symmetry.
In the IC industry there is continuing demand for miniaturization of the wafer patterns. This is leading to a reduction in the dimensions of electrical components produced on silicon wafers. Therefore, there is a need for improving the detection capability of the inspection machines by improving resolution, signal to noise ratio and contrast. Moreover, the quantity of inspection data is increasing with the increase in resolution and therefore there is a need for inspection machines with a higher throughput.
Recently a novel scanning system was described in a U.S. Pat. No. 6,310,710 to Shahar et al., entitled xe2x80x9cHigh-Resolution Reading and Writing Using Rotating Beams and Lenses Rotating at Equal or Double Speedxe2x80x9d. The aforementioned system provides better resolution and higher throughput as compared to other scanning systems. Accordingly, the aforementioned system is very attractive to the IC industry for fulfilling the current and future demands in the field of silicon wafers inspection. The scanning system described in the aforementioned system is a circular scanner including at least one scanning beam that is operated with or without confocal mode. The circular scanner produces a scan along a circular path and therefore does not have the symmetry of a Cartesian coordinate system. Therefore, performing the Cell-to-Cell or Die-to-Die comparison method with the rotating microscope leads to a mismatch between the circular paths of the scanner and the orthogonal symmetry of the silicon wafers. Therefore, large quantities of inspection data need to be stored relating to the area of several dies. Therefore a very large memory-volume is required, which makes the use of the circular scanner impractical, in spite of all its advantages.
Reference is now made to FIG. 1 and FIG. 2. FIG. 1 is a side view of a scanning arrangement 5 configured to perform circular scanning paths that is constructed and operable in accordance with the prior art. FIG. 2 is a plan view of scanning arrangement 5. Scanning arrangement 5 includes a circular scanner 6, a stage 19 and a drive mechanism 21 (not shown). Circular scanner 6 includes a spindle 8, a polygon 10, a disk 12 and at least one scanning head 14. Circular scanner 6 has an axis of rotation 15 about which the rotating elements of circular scanner 6 rotate. Spindle 8 rotates polygon 10 at a rate W about axis of rotation 15. Spindle 8 rotates disk 12 at a rate 2 W about axis of rotation 15. Disk 12 carries scanning head 14. Therefore, scanning head 14 performs a circular scanning motion about axis of rotation 15 due to the rotation of disk 12 over an inspection area 18 of a sample. The sample containing inspection area 18 is mounted on stage 19. Drive mechanism 21 is configured to provide relative linear movement between stage 19 and axis of rotation 15 in a direction perpendicular to axis of rotation 15 in order to enable circular scanner 6 perform an area scan. Circular scanner 6 also includes a light source 20, an optical apparatus 24, an auto focus system 26 and a light detector 30. The optical path of a light beam 16 originates from light source 20. Light beam 16 is transmitted from light source 20 through optical apparatus 24 and optional auto focus system 26 to polygon 10. Light beam 16 is reflected by the surfaces of polygon 10 along a path 32 to scanning head 14 (FIG. 2). It is shown in the prior art that path 32 is equivalent to a path 34 and therefore circular scanner 6 preserves the length of the optical path of light beam 16 at all times (FIG. 2). Light beam 16 is projected to a point 28 which is on inspection area 18 by scanning head 14. Light beam 16 is reflected from inspection area 18 via scanning head 14, polygon 10, auto focus system 26 and optical apparatus 24 to light detector 30. Light beam 16 is a single beam or a collection of multiple beams. The scanning path produced by light beam 16 is referred to as a scanning swath. If light beam 16 includes a collection of multiple beams, then the scanning path produced by each multiple beam is referred to as a curved scanning path. Therefore, if light beam 16 includes a collection of multiple beams, there will be a plurality of curved scanning paths per scanning swath.
Reference is now made to FIG. 3, which is a schematic plan view of a scanning pattern 35 produced by scanning arrangement 5 in accordance with the prior art. This particular example describes a system having four scanning beams included within light beam 16. Circular scanner 6 scans inspection area 18 by moving a light spot 38, a light spot 40, a light spot 42 and a light spot 44 along a curved scanning path 46, a curved scanning path 48, a curved scanning path 50 and a curved scanning path 52 respectively. Inspection area 18 has Cartesian symmetry is composed of a periodic pattern which is schematically shown as a plurality of blocks 54.
Reference is now made to FIG. 4, which is a schematic plan view of an area scanning pattern 56 produced by scanning arrangement 5 in accordance with the prior art. In order to produce area-scanning pattern 56, relative linear motion between the axis of rotation 15 (FIG. 1) and stage 19 (FIG. 1) is introduced between each scanning swath performed by circular scanner 6. This particular example also describes a system having four scanning beams included within light beam 16. Area scanning pattern 56 includes a plurality of successive scanning swaths 58, 60, 62, 64 and 66 produced by rotating disk 12. Relative movement is generated between axis of rotation 15 and stage 19 between the production of each of scanning swaths 58, 60, 62, 64 and 66. It is seen that a comparison between successive scanning swaths 58, 60, 62, 64 and 66 is not useful since they do not scan similar physical areas on inspection area 18. The only way to perform a comparison is by storing large amounts of information and to detect the matching physical areas stored in the information.
There is therefore a need for a method to inspect surfaces having a periodic pattern with a first type of symmetry, such as an IC wafer having the symmetry of a Cartesian coordinate system, using a scanner having a second type of symmetry, such as circular scanner, while keeping the memory requirements and processing power at a reasonable level.
The present invention is a method for inspecting surfaces having a periodic pattern.
According to the teachings of the present invention there is provided, a method to compare similar physical areas of an inspection area of a sample using a scanning arrangement, the inspection area having a periodic pattern having a repeat vector, the scanning arrangement having a stage configured for mounting the sample thereon, the scanning arrangement having a drive mechanism and at least one circular scanner, the circular scanner having at least one scanning head and an axis of rotation, the scanning head performing a circular scanning motion about the axis of rotation, the drive mechanism configured to provide relative movement between the stage and the axis of rotation, the method comprising the steps of: (a) scanning the inspection area by a combination of circular scanning of the scanning head and by generating relative movement between the stage and the axis of rotation so as to generate a scanning pattern which includes a plurality of curved scanning paths wherein pairs of the curved scanning paths are related by an integer multiple of the repeat vector; and (b) comparing at least one of the pairs of the curved scanning paths by a pixel to pixel comparison.
According to a further feature of the present invention: (a) the at least one circular scanner is implemented as at least two circular scanners; and (b) the axes of rotation of the at least two circular scanners are separated by a multiple of the repeat vector.
According to a further feature of the present invention: (a) the axes of rotation of the at least two circular scanners are connected by a line which is parallel to the repeat vector; and (b) the relative movement between the stage and the axes of rotation is generated in a direction which is parallel to the repeat vector.
According to a further feature of the present invention: (a) the axes of rotation of each of the at least two circular scanners are connected by a line which is parallel to the repeat vector; and (b) the relative movement between the stage and the axis of rotation is generated in a direction which is perpendicular to the repeat vector.
According to a further feature of the present invention the axes of rotation of the at least two circular scanners are separated by a distance substantially equal to a diameter of each of the curved scanning paths.
According to a further feature of the present invention the axes of rotation of the at least two circular scanners are separated by a distance less than a diameter of each of the curved scanning paths.
According to a further feature of the present invention: (a) the inspection area includes a plurality samples which are substantially identical, each of the samples having a key point, the repeat vector being the separation between key points of the samples; and (b) the method further includes the step of mounting the samples on the stage such that the key points of the samples are directed in the same direction.
According to a further feature of the present invention the step of scanning the inspection area is performed by scanning the inspection area by generating relative linear movement between the stage and the axis of rotation while at the same time performing circular scanning of the scanning head so as to generate a scanning pattern which includes a plurality of curved scanning paths wherein pairs of the curved scanning paths are related by an integer multiple of the repeat vector.
According to a further feature of the present invention the relative linear movement is at constant velocity.
According to a further feature of the present invention a first integer multiplied by a time taken to generate one of the curved scanning paths is substantially equal to a second integer multiplied by a time taken to advance the stage relative to the axis of rotation by a distance equal to the length of the repeat vector.
According to a further feature of the present invention the second integer is equal to one.
According to the teachings of the present invention there is also provided, a method to compare similar physical areas of an inspection area using a circular scanner, the inspection area including a plurality of samples, the samples being substantially identical, the circular scanner having a stage apparatus, a drive mechanism, at least one scanning head and an axis of rotation, the at least one scanning head performing a circular scanning motion about the axis of rotation, the stage apparatus having at least two stage portions, the drive mechanism configured to provide relative movement between each of the stage portions and the axis of rotation, the method comprising the steps of: (a) mounting the samples on the stage apparatus such that there is one of the samples per one of the stage portions such that the samples are disposed symmetrically around the axis of rotation; (b) scanning at least part of the samples by employing the scanning head to perform a substantially circular scanning path; (c) comparing at least two best matched curved scan paths on the substantially circular scanning path by a pixel to pixel comparison; and (d) moving the samples relative to the axis of rotation such that the samples maintain a symmetrical disposition around the axis of rotation.